1. Field of the Invention
The present invention relates to an optical integrated device primarily used in optical communication, and a method of manufacturing the optical integrated device.
2. Description of the Related Art
With advancement of optical communication technique, there has been a growing demand for high performance, downsizing, and low power consumption of an optical device. An optical integrated device, in which optical devices are highly integrated, is expected to be a solution to such a demand.
A deep ridge waveguide is expected to be a waveguide structure of an optical integrated device using a semiconductor (refer to Patent Reference). FIG. 32 is a cross sectional view schematically showing a structure of a conventional deep ridge waveguide.    Patent Reference: Japanese patent publication No. 2003-207665
As shown in FIG. 32, the deep ridge waveguide has a structure, in which a lower clad layer 71, formed as a portion of a substrate 70 projecting from a surface of the substrate 70 by a projection height d5 and extending along the surface of the substrate 70, a core layer 72, and an upper clad layer 73 are sequentially formed in this order on the substrate 70. A refractive index of the core layer 72 is greater than those of the lower clad layer 71 and the upper clad layer 73, and extremely greater than that of the air. Accordingly, light propagating in the core layer 72 is confined due to a difference in the refractive indexes between the core layer 72 and the lower clad layer 71 or the upper clad layer 73 in a thickness direction, and further due to a large difference in the refractive indexes between the core layer 72 and the air in a lateral direction.
The structure can be formed through sequentially growing the core layer 72 and the upper clad layer 73 on the substrate 70, and thereafter etching to a depth reaching the substrate 70, to leave the portion to be the waveguide.
It is preferable that the waveguide of the optical integrated device is a single mode waveguide for allowing light to propagate only in a fundamental mode. When a deep ridge waveguide formed of a semiconductor is designed to be a complete single mode waveguide, a width of the waveguide (mesa width) needs to be far less than 1 μm, thereby making it difficult to produce the waveguide. Therefore, in an actual design, the mesa width is often set to around 2 μm, thereby allowing the waveguide to be a multimode waveguide in which some higher order modes can propagate.
Patent Reference has disclosed a design to suppress light propagation in higher order modes for a deep ridge waveguide having a mesa width with which the waveguide acts as a multimode waveguide. In the deep ridge waveguide, the core layer and the upper and lower clad layers are configured such that a mode refractive index of a fundamental mode is greater than a refractive index of the substrate, and a mode refractive index of higher order modes is less than the refractive index of the substrate. Further, the projection height of the lower clad layer from the substrate is small. Accordingly, the higher order mode light can be attenuated by being radiated into the substrate, and therefore the light propagation in higher order modes can be restrained.
In the optical integrated device, in order to properly connect each component of the optical integrated device, it is necessary to use not only a straight waveguide but also a bending waveguide curved along a surface of the substrate. However, in the bending waveguide designed to suppress light propagation in higher order modes, an excessive propagation loss (bending propagation loss) of the fundamental mode occurs due to the bending.
A relation between a radius of the curvature of the bending waveguide and the bending propagation loss of the fundamental mode will be explained below. An electromagnetic field simulation is performed for the deep ridge waveguide designed such that the higher order mode light is radiated into the substrate as disclosed in Patent Reference.
In the simulation, a thickness of the core layer is set to 0.30 μm, a projection height of the lower clad layer from the substrate is set to 0.5 μm, a thickness of the upper clad layer is set to 2.50 μm, and the mesa width is set to 2.0 μm. Further, a refractive index of the core layer is set to 3.31, a refractive index of the upper clad layer and the lower clad layer is set to 3.17, and a refractive index of the medium (air) around the mesa structure is set to 1.00. Under such design parameters, a mode refractive index of the fundamental mode in the waveguide is greater than the refractive index of the substrate, and a mode refractive index of higher order modes is less than the refractive index of the substrate, whereby the higher order mode light is radiated into the substrate.
FIG. 33 shows a result of the calculation of the bending propagation loss of the fundamental mode as a function of a radius of the curvature in the conventional deep ridge waveguide. As shown in FIG. 33, in the conventional deep ridge waveguide, the bending propagation loss increases exponentially with a decrease in the radius of the curvature for both the TE mode and the TM mode. The bending propagation loss is 2 dB/mm in the case of the radius of the curvature of 200 μm, and reaches as large as 8 dB/mm for the case of the radius of the curvature of 100 μm.
As described above, in the deep ridge waveguide designed such that the higher order mode light is radiated, the bending propagation loss of the fundamental mode is not negligible. The bending propagation loss becomes extremely great when the radius of the curvature is small, thereby making it difficult to downsize or highly integrate an optical integrated device.